The invention relates generally to electrical devices and, more specifically, in certain embodiments, to ground-fault sensors.
A variety of electrical systems monitor ground faults with a ground-fault sensor. Typically, these sensors detect currents between the electrical system and ground, as might occur when the electrical system charges some object outside the system. For instance, ground currents might occur as a frayed wire contacts an electrical conductor outside of the electrical system. In the event that current flows to or from ground, it is useful to detect the ground fault, so electrical power can be removed from the system before the ground current damages the electrical system or other devices.
To identify a ground current, some ground-fault sensors detect electromagnetic fields that arise during a ground fault. A ground fault typically produces a net current flow to, or from, one side of a circuit, as electrons leave or enter the system via the ground fault. This is referred to as a differential-mode current. Differential-mode currents are distinguished from common-mode currents, which occur during normal operation as the electrons that flow in one side of the circuit are balanced by electrons that flow out the other side. A typical result of a differential-mode current is a detectable electromagnetic signature. The signature, however, can be difficult for conventional ground-fault sensors to detect, because the electromagnetic fields produced by normal, common-mode currents can create noise that is difficult to distinguish from the signature of a differential-mode current.
Another issue with conventional ground fault sensing techniques resides in the mechanisms used to detect unbalanced current flow by monitoring fields surrounding conductors. For example, ground faults in three-phase systems can be detected by passing three phase conductors through a common toroidal current sensing coil. The positions of the conductors, however, are rarely the same, leading to slight, but detectable imbalances in the magnetic flux that induces the measured current in the sensing coil. Devices, such as iron rings, can be used in conjunction with such arrangements to help spread or distribute the flux, but are not fully effective at eliminating the effect.
There is a need, therefore, for improved methods and arrangements for detecting ground faults. There is a particular need for an arrangement that can be used in connection with three-phase electrical equipment.